1. Field
Example embodiments relate to a multiple reflection layer electrode of a semiconductor device, a compound semiconductor light emitting device having the same and methods of fabricating the same. Other example embodiments relate to a multiple reflection layer electrode having improved thermal stability and ohmic contact characteristics, a compound semiconductor light emitting device having the same and methods of fabricating the same.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a structure of a conventional compound semiconductor light emitting device (LED) 50 and a conventional p-type electrode 30 formed on a p-type nitride semiconductor layer 16. FIG. 2 is a photograph showing the surface of the compound semiconductor LED on which the p-type electrode 30 is annealed, and FIG. 3 is a scanning electronic microscope (SEM) cross-sectional photo showing agglomeration of the annealed p-type electrode 30 and voids 32 formed in the p-type electrode 30 as a result of agglomeration.
Referring to FIG. 1, the conventional semiconductor LED 50 may include an n-type nitride semiconductor layer 12, an active layer 14, and a p-type nitride semiconductor layer 16, which are sequentially formed on a sapphire substrate 10, an n-type electrode 20 formed on one side of the n-type nitride semiconductor layer 12, and a p-type electrode 30 formed on the p-type nitride semiconductor layer 16. If forward voltages are applied to LED electrodes, for example, the n-type electrode 20 and the p-type electrode 30, electrons in a conduction band of the active layer 14 may be recombined with holes in a valence band and light may be emitted from the active layer 14 due to energy corresponding to a band gap, which is the energy difference between the valance band and the conduction band. Light emitted from the active layer 14 may be reflected by the p-type electrode 30 and may be emitted to the outside of the semiconductor LED 50 through the sapphire substrate 10.
In an LED in which light generated from the semiconductor LED 50 is not directly emitted onto the sapphire substrate 10, but is reflected by the p-type electrode 30 and emitted through the sapphire substrate 10 (hereinafter, referred to as a flip-chip LED), because the p-type electrode 30 may reflect light, the p-type electrode 30 may be formed of a conductive metal having increased reflectivity, e.g., Ag.
A semiconductor having a relatively large direct bandgap energy (about 2.8 eV or more) may be essential for blue light emission. Semiconductor devices, which emit a blue or green light by primarily using a Group II-VI ternary system material, have been developed. However, due to a relatively short operating time, there are problems in applying semiconductor devices. Recently, semiconductor devices for blue light emission have been studied in Group III-V semiconductors. Among them, Group III nitride (for example, a compound related to GaN) semiconductors may be relatively stable to optical, electrical, and thermal stimulus and may have an increased luminous efficiency.
As illustrated in FIG. 1, in an LED that uses a Group-III nitride semiconductor, e.g., GaN, as a semiconductor light emitting device, for improvement in light extraction efficiency, the p-type electrode 30 may be formed of a metal having increased reflectivity, e.g., Ag, on the p-type nitride semiconductor layer 16. In order to form the p-type electrode 30 on the p-type nitride semiconductor layer 16, an electrode may be deposited on the p-type nitride semiconductor layer 16 and then, annealing may be necessary for reduction in resistance.
However, in general, a surface energy of a nitride semiconductor and a surface energy of a metal material, e.g., Ag, used in forming the p-type electrode 30 may be different from each other. Due to the difference in surface energies, agglomeration may occur in the p-type electrode 30 during annealing, as shown in the photographs of FIGS. 2 and 3. FIG. 2 is a plan-view of a captured image of the n-type electrode 20 and p-type electrode 30 in which surface agglomeration occurs after annealing, and FIG. 3 is an SEM cross-sectional photo of the p-type electrode 30 in which agglomeration occurs after annealing. As shown in FIGS. 2 and 3, a plurality of voids 32 may be formed at an interface between the p-type nitride semiconductor layer 16 and the p-type electrode 30. When agglomeration occurs in the p-type electrode 30, a plurality of voids 32 may be formed. As a result, reflectivity of the Ag electrode 30 may be lowered and an optical output of the entire LED may be reduced.